Image-projection optical system with cylindrical screen and stop means



Sept. 20, 1949. 2,482,566

E. H. TRAUB IMAGE-PROJECTION OPTICAL SYSTEM WITH CYLINDRICAL SCREEN AND STOP MEANS Fil'ed June 16, 1945 Patented Sept. 20,1949

IMAGE-P301 ECTION WITH CYLINDRIGAL STOP IVIEANS OPTICAL SYSTEM SCREEN AND Ernest Traub, Philadelphia, Pa., assignor, by

mesne assignment Philadelphia, Pa., vania to a corporation of Pennsyl- Philco Corporation,

Application June 16, 1945, Serial No. 599,927

This invention relates to image-projection optical systems employing a cylindrical optical element, and .more'particularly to systems of the type in which an image-producing device is disposed in the path of projection or light transmission. The term cylindrical is here used in the optical sense to mean that the said element is curved in some fashion about a single axis, i. e. itis curved only in one direction, as distinguished, for example, from a spherical element.

In its broad aspect, the invention is generally applicable to image-projection systems employing a cylindrical optical element. In its more limited aspects, the invention is primarily intended for use in such an image-projection system adapted to be employed in television receiving apparatus. One example of such a system is a reflective system employing a. Schmidt correcting plate, in which system the image-pro-' ducing' picturetube is disposed along the optical axis of the system and the correcting plate surrounds the neck of the tube. However, the invention is not limited to a reflective system, but is applicable as well to systems involving refraction only, as will be shown later.

The use of a cylindrical optical element in an image-projection system is highly desirable in some instances. For example, such an element may be employed to advantage as an imageviewing screen, as described more fully hereinafter. When such an element is employed, however, it accentuates variations in overall brightness of the reproduced image as viewed from diiierent positions in the viewing plane. The reason for this will be shown later.

The principal object of the present invention is to reduce, or substantially eliminate, such variations in the overall brightness of the reproduced image. 7 Another object 01 the invention is to provide a novel optical stop means for the stated purpose.

Still another object of the invention is to provide a novel image-projection system for use in television receiving apparatus, in which system the overall brightness of the reproduced image is substantially uniform throughout the viewing plane, with the result that an observer may view the reproduced image from any point in the viewing plane 'without' noticeable variation in overall brightness of the reproduced image.

e Other objects and features of the invention will become apparent as the description proceeds. -Inthe accomp yingdrawinga' Claims. (c1. 178-'i.5)Y

' Figs. 1a, 1b and 1c illustrate schematically the brightness distribution curves obtained with difierent typesoiaperture stops in a system employing, a. cylindrical optical element;

Fig. 2 is a similar illustration, on a larger scale, illustrating one form of the novel aperture stop means employed according to the present invention, and also illustrating the brightness distribution curve obtained;

Fig. 3 is a similar illustration of another form of the stop means; 7'

Fig. 4 illustrates an image-projection system of the reflective type, embodying the present invention; and

Fig. 5 illustrates an image-projection system of the refractive type, embodying the invention.

The invention may best be described by first referring to Fig. 4 which may be taken as typical ofa system employing a cylindrical optical element as above mentioned. In this instance, the cylindrical .element serves. as the image-viewing screen.w While the present invention'is primarily concerned with .asystem in which such an element is thus employed, it is not limited thereto but is applicable in anyiinstance in which a cylindrical element forms part of the image-reproducing means generally. The system illustrated inFig. 4 is adapted for use in a television receivingapparatus, and it comprises a picture tube T, a spherical reflectoror mirror M, a Schmidt correcting plate C, and a cylindrical image-viewing screen S. The image formed on the screen tof tube T is reflected by the reflector M onto the cylindrical screen S which may be either a lens raster screen or a simplescreen acting as a fleld lens. The cylindrical screen S is adapted to be viewed from. a position in the, viewing plane represented at P. The optical axis .of the system is indicated bythe dot and dash lines. The correcting plate C serves to correct for sphericaleaberration of the reflector M, as well understood by those skilled in the art. For the present, the-elements s1 and s2 may be disregarded. V

The cylindrical screen S, which is illustrated withits axis disposed horizontally, is preferably of the type disclosed'and claimed in copending application Serial No. 651,064, filed 'March 1, 1946, now abandoned. It may consist ofa sheet of stainless steel having fine scratches or marks running vertically. Aspreviously mentioned, the useof a. cylindrical screen is very desirable. Such a' screen is highly efiicient in the reproduction of projected enlarged images because it ofisets, or. -compensates for, in; major part the loss "in brightness normally Such a screen is also distinctive as respects the apparent brightness of the reproduced image in relation to the aperture of the optical system, as may be seen from the following comparison with an ordinary screen. For ordinary screens, the apparent where BS is apparent brightness of theimage, Br

brightness is expressed 'by the is the primary image brightness, F is the numeri-- cal aperture of the system, and M is the magnification power of .the system. It'willrbe seen that the apparent brightness is dependent uponboth horizontal and vertical aperture, since the numerical aperture .F is determined. by the entire aperture area;

For a cylindrical. screen, however, arranged so that. its axis is horizontal, the apparent brightness of the image is expressed by the following equation:

where 35 is apparent brightness of the image, Bi and the other quantities are as previously mentioned. It will be seen that in this case,v the apparent brightness of. the image is independent of vertical aperture and is dependent only upon horizontalv aperture. 7

Notwithstanding its desirability, the cylindrical image-reproducing element or screen causes undesired variation in overall brightness of the image as viewed from vertically different positions in .the viewing plane, especially when the screen is employed in a system of the type in which. an image-producing device is disposed in the path of. projection, as in the system of Fig. 4. This is due to the fact that, with such a screen, the apparent brightness of the imageisvery substantially aiiected by variation of the horizontal aperture, as is evident from Equation 2 above.

The undesirable variation in overall brightness produced. by such a screen may be'envisioned with the aid of Figs. la to 10. Fig. la, depicts-the ideal condition which would obtain if the aperture (2.1 were square orrectangular, i. e. of uniform hori zontal dimension. As shown by the brightness distribution curve 01, the overall brightness of the imagewould be constant throughoutthe'vertical height of the viewing plane. It is customary; however, to employ circular or round optical elements, and therefore the condition of Fig. la does not obtain in practice.

Fig. lb shows the variation in overall image brightness due to variation of the horizontaP dimension of a circular or round aperture (12. The brightness distribution curve 02' shows that the image is'brightest' centrally of the vertical height of the viewing plane and decreases for other positions in said plane. This is due to thefact that the horizontal dimension of the aperture isgreatest centrally of its height and decreases above and below the center. This efiect'is accentuated by thecylindrical screen for the reason previously stated. V V

Whilethe condition of Fig. 1b may be tolerable,

vary sharply and the undesired effect becomes intolerable when the horizontal dimension of the aperture is further varied by central masking due to the presence of the image-producing device in the path of projection, asin Fig. 4. The condition which then obtains is depicte'dirLFig. 1c, wherein the shaded area V represents central masking of the aperture a3. Due to accentuation of the above-mentioned e fiect by the cylindrical screen, the overall brightness of the image is caused to substantially at difierent posivertical height of the viewing As shown by curve 03, the image appears tions along the plane.

brightestwhen viewed from either of two vertically spaced positions in the viewing plane, and its brightness is greatly diminished when viewed from, an intermediate position. This is evidenced by the two distinct humps in curve cs.

"Inaccordance with the present invention, this objection is overcome by employing a novel stop means of. the character illustratedin Fig 2. In the illustration, it is assumed that the central masked area. V is circular. in shape, and the stop means is designed. accordingly;v but it is to be understood thatthestop means should be designed in any instance according to the shape of the masked area. The novel stop means is characterized in that it provides a horizontal aperture of substantially constant width, thereby eliminating .the' objectionable brightness variation above mentioned.

Referring to Fig. 2, the stop means illustrated comprises non-transmissive elements s1 and 82, having arcuaterecesses r1 and rz aligned horizontally opposite each other, the radius of eachrecess being approximately that of the masked area V, with the distanced equalto the sum of. the distances c1 and en. Accordingly, the elements s1 and 82' form a light-transmission areaof substantially constanthorizontal width, with the result that auniform' brightness curve is'obtained, as shown at 04in Fig, 2, which is a close approximation of the idea brightness curve shown at 01 inFigl'm;

Referring again toFigMl, the novel stop means of the present invention may be conveniently formed as a part of thecorrecting plate C by painting or otherwise forming the stopelements s1 and .92 directly on the correctin plate as illustrated. However, the stop means may take the form of a separate element, and it may be disposed either in frontof or behind the correctingplate, so long as it is interposed in the-projection path between elementsM and S in a position to provide the'apert'ure 0f the system.

As previously 1 noted,- the cylindrical image reproducing element S is so'disposed' that its axis is horizontal, and while such disposition of element S is 'preferable'and the foregoing description is with reference thereto, no limitation isintended in this respect. In any event, however, the stop means provided by the present inven tion is so positioned in relation to the cylindrical element'that the'light-transmission area formed by the stop means is of substantially constant width in the direction of the axis; of the cylindrical element.

In Fig. 5-, there is shown a refractive system in-which the picture tube Ti is disposed in the path'of projection, and the image produced on the'face-or screen ii of the tube is projected through optically finished portions :1; of the tube and is focused onto'the cylindrical element or screen S1 by means of a lens L- surrounding the neck of the tube. The 'stopmeana'provided by the present invention, may compriseopaque and transparent areas formed on the correcting plate C1, as previously described. Thus, it will be seen that the invention is applicable to refractive systems as well as to reflective systems.

In a system of the character described, the tube mounts, wiring, etc., may cause additional masking, the deleterious effect of which may be overcome in the manner illustrated in Fig. 3. If these elements which cause the additional masking are arranged vertically, the additional masking is along a vertical strip, as shown by the shaded area V1 in Fig. 3. In this instance, the additional masking may be compensated for by maintaining equality between the sums of the distances (Z1 and d2, and c1 and ez, thereby producing the uniform brightness distribution shown by the curve 05.

While the invention has been described with reference to the illustrated embodiments, it is not limited thereto but is susceptible to further embodiments and modifications within the scope of the appended claims.

I claim:

1. In an image-projection system, an imageproducing device a spherical mirror arranged to project the image, means including a cylindrical element for reproducing the image, a correcting plate interposed in the path of light projection between said mirror and said element, said element tending to accentuate variations in overall brightness of the image, as viewed from different positions in a viewing plane, caused by diminution of the systems aperture area in the direction of the axis of said element, and stop means associated with said correcting plate and adapted to render the width of the aperture area substantially constant in the direction of the axis of said element, whereby substantially to eliminate said variations.

2. In a picture projection system for a television receiver, a picture tube for producing the picture image to be projected, a spherical mirror arranged to project the image, said tube being disposed in the path of projection and causing masking of the systems aperture, a correcting plate surrounding the neck portion of said tube and adapted to correct for spherical abberation of said mirror, means including a cylindrical element for reproducing the image, said element tending to accentuate variations in overall brightness of the image, as viewed from different positions in a viewing plane, caused by diminution of aperture area in the direction of the axis of said element, and stop means on said correcting plate adapted to compensate for said masking and to render the width of the aperture area substantially constant in the direction of the axis of said element, whereby substantially to eliminate said variations.

3. In an image-projection system, an imageproducing device disposed in the path of projection and causing masking of the systems aperture, a spherically surfaced converging means arranged to project the image, means including a cylindrical element for reproducing the image, said element tending to accentuate variations in overall brightness of the image, as viewed from difierent positions in a viewing plane, caused by 4. Inan image-projection system, an imageproducing device disposed in the path of projection and causing masking of the systems aperture, a spherically surfaced converging means arranged to project the imaga'means including a cylindrical element for reproducing the image,

1 said element tending to accentuate variations in overall brightness of the image, as view-ed from different positions in a viewing plane, caused by diminution of aperture area in the direction of the axis of said element, a light-transmissive member disposed in the path of projection, said member being so positioned and so shaped as to provide the aperture for said system, and nontransmissive elements on said member adapted to render the width of the aperture area substantially constant in the direction of the axis of said cylindrical element, whereby substantially to eliminate said variations.

5. In a picture-projection system for a television receiver, a picture tube for producing the picture image to be projected, a spherically surfaced converging means arranged to projectthe image, said tube being disposed inthe path of projection and causing masking of the systems aperture, means including a cylindrical element for reproducing the image, said element tending to accentuate variations in overall brightness of the image, as viewed from different positions in a viewing plane, caused by diminution of aperture area in the direction of the axis of said element, a light-transmissive member disposed in Y the path of project-ion, said member being so positioned and s0 shaped as to provide the aperture for said system, and non-transmissive elements on said member adapted to render the width of the aperture area substantially constant in the direction of the axis of said cylindrical element, whereby substantially to eliminate said variations.

ERNEST H. TRAUB.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES The American Photo-Engraver, vol. 21, No. 10, 1929, page 937-951; article by J. S. Mertle, page 944, cited.

Certificate of Correction Patent No. 2,482,566 September 20, 1949 L ERNEST H. TRAUB It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 33, strike out where B is apparent brightness of the image, B and insert instead where F is the horizontal aperture of the system;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 24th day of January, A. D. 1950.

THOMAS F. MURPHY,

Assistant Uommiasz'oner of Patents. 

